Determination of the methanogenic potential of an apple processing wastewater treatment system

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(1)DETERMINATION OF THE METHANOGENIC POTENTIAL OF AN APPLE PROCESSING WASTEWATER TREATMENT SYSTEM. CINDY PAULSEN Thesis approved in partial fulfilment of the requirements for the degree of. MASTER OF SCIENCE IN FOOD SCIENCE In the Department of Food Science, Faculty of AgriSciences University of Stellenbosch. September 2006. Study Leader: Co-study Leaders:. Dr. G.O. Sigge. Professor T.J. Britz & Ms. E. Muller.

(2) ii. DECLARATION I, the undersigned, hereby declare that the work contained in this thesis is my own original work and has not previously in its entirely or in part been submitted at any university for a degree.. Cindy Paulsen. Date.

(3) iii. ABSTRACT The food and beverage industry generates large volumes of wastewater annually. The disposal of factory effluent from the fruit processing industry has always been a cause of concern to both the fruit processors and controlling bodies responsible for effluent management.. Traditional disposal of wastewater into sewerage works has become. undesirable due to its economical and environmental impacts.. Therefore, on-site. anaerobic treatment of wastewater has received considerable interest due to lower capital outlays and energy recovery possibilities. Thus, the aim of this study was to establish an operational treatment profile for an anaerobic pond system treating fruit-processing wastewater.. The specific activity of the microbial populations was also monitored to. determine the effect of the fruit processing seasons (peak and off-peak season). The biogas production potential at various temperatures was also assessed to determine the viability of methane recovery. The influence of the processing and environmental conditions on the ponds’ performance was established by monitoring various process parameters.. The results. showed that the chemical oxygen demand (COD) levels decreased during the off-peak season but the pond pH remained relatively stable between 6.0 and 6.4 during the entire year. Pond alkalinity was found to be dependent on the regular lime dosing to maintain the necessary alkalinity. The volatile fatty acid (VFA) concentrations indicated that the microbial populations of the pond were functioning well. However, a decrease in microbial activity and VFA concentrations were observed at the lower temperatures during the winter months. The temperature profile of the pond showed that the pond temperature was impacted by the fluctuations in the ambient air temperature. The general trend established by the operational treatment profile clearly showed the impact of the peak and off-peak season. The sludge activity of the anaerobic pond was evaluated to determine the effect of the apple-processing peak and off-peak season on the specific activity of the acidogenic and methanogenic populations within the sludge. An activity test using four different test media was used during the activity assays. Sludge samples were taken at four different sampling positions across the pond’s sludge bed. The sludge was also subjected to a biogas formation study, which was designed to simulate pond conditions on laboratory.

(4) iv. scale in order to evaluate the biogas production potential of the anaerobic pond. The cumulative biogas volume and total CH4 composition showed little or no difference between the four sludge sampling sites. A major difference was found between the activity of the microbial populations during the peak and off-peak seasons. The overall trend regarding the biogas production rate (SB) and the methane production rate (SM) values showed an increased activity during peak-season and a decreased activity during off-peak season. For the biogas formation test the highest incubation temperature (25°C) resulted in the most biogas being produced, followed by 18°C, and with 10°C resulting in the lowest biogas volume. The biogas formation tests indicated that microbial activity and therefore biogas production was dependent on especially favourable temperature conditions. The pond and activity of the microbial populations are therefore influenced by factors like environmental changes such as decreased air temperatures and substrate changes such as decreased COD concentrations during the off-peak season. This in turn influences the rate of biogas production as well as the methane production rate. The theoretical CH4 calculations and estimates based on the results obtained during the biogas formation tests indicated that CH4 recovery from the anaerobic pond would definitely be a worthwhile consideration.. If it were assumed that the estimated CH4. volumes (based on only 15% of the pond volume for practical reasons) obtained could be applied as an energy source, the minimum yearly savings in coal usage would amount to about R 665 000. This study was valuable in evaluating the factors such as pond conditions, pond activity and air temperatures and the effect on the biogas production potential as well as more importantly, CH4 production for the purpose of energy recovery..

(5) v. UITTREKSEL Die voedsel en drank industrie produseer jaarliks groot volumes uitvloeisel. behandeling. en/of. verwydering. van. uitvloeisel. afkomstig. van. die. Die vrugte. verwerkingsindustrie is ‘n bron van kommer vir die vrugteverwerker asook beheerliggame wat verantwoordelik is vir die bestuur van besoedelde uitvloeisel.. Tradisionele. verwydering van uitvloeisel deur munisipale suiweringswerke is deesdae ongewens as gevolg van die ekonomiese en omgewingsnadele wat dit inhou. Dit het veroorsaak dat behandelingsmetodes soos anaerobiese behandeling, wat op die verwerkingsperseel onderneem kan word, baie gewild raak. Hierdie behandelingsmetode het baie voordele in terme van laer kapitaal belegging en energie herwinningsmoontlikhede. Dus, die doel van die studie was om ‘n bedryfsprofiel op te stel van ‘n anaerobiese dam sisteem wat uitvloeisel behandel vanaf ‘n vrugte verwerkingsfabriek. Die spesifieke aktiwiteit van die mikrobiese populasies is ook gemonitor om die effek van die vrugte seisoene (in en afseisoen) te bepaal.. Die biogas produksie potensiaal by verskeie temperature is ook. ondersoek om die lewensvatbaarheid van metaan herwinning te bepaal. Die invloed van die verwerkingseisoene en omgewingsfaktore is bepaal deur verskeie bedryfsparameters te monitor.. Die resultate het getoon dat die chemiese. suurstof vereiste (COD) gedaal het tydens die af-seisoen, maar dat die pH van die dam relatief stabiel gebly het tussen 6.0 en 6.4 deur die verloop van die jaar. alkaliniteit. was. afhanklik. van. gereelde. kalk-dosering. om. die. Die dam. noodsaaklike. alkaliniteitsvlakke te handhaaf. Die kort ketting vetsuur (VFA) konsentrasie het getoon dat die mikrobiese populasies goed funksioneer in die dam. Alhoewel daar ‘n afname in die mikrobiese aktiwiteit was sowel as die VFA konsentrasie tydens die wintermaande. Die temperatuur profiel het getoon dat die damtemperature beïnvloed word deur lugtemperatuur fluktuasies. Die algehele bedryfsprofiel het duidelik die impak van die prosesseringseisoene getoon. Die slykaktiwiteit van die anaerobiese dam was ge-evalueer om die impak van verwerkingseisoene op die spesifieke aktiwiteit van die asidogene en metanogenepopulasies te bepaal. ‘n Aktiwiteitstoets is gebruik met vier verskillende toets substrate. Die slykmonsters was geneem by vier verskillende posisies in die dam. Die slykmonsters is ook gebruik in ‘n biogas formasie studie, wat die damtoestande gesimuleer in op klein.

(6) vi. skaal. Hierdie studie was uitgevoer om die biogas produksie potensiaal van die dam vas te stel. Die kumulatiewe biogas volume en totale metaan inhoud van die vier slykmonsters het slegs klein verskille getoon. Daar was hoewel ‘n groot verskil tussen die aktiwiteit van die mikrobiese populasies tydens die in- en af-seisoen.. Die algehele tendens in die. aktiwiteit het getoon op ‘n afname in die SB en SM waardes tydens die af-seisoen en ‘n toename tydens die in-seisoen. Die biogas formasie toetse het aangedui dat die hoogste inkubasietemperatuur (25°C) die grootste hoeveelheid biogas oplewer, terwyl die laagste inkubasie temperatuur die kleinste hoeveelheid opgelewer het. Hierdie biogas formasie toetse het getoon dat die mikrobiese aktiwiteit en dus die biogas produksie potensiaal afhanklik was van gunstige temperatuur toestande. Dus word die dam en die aktiwiteit van die mikrobiese populasies grootliks beïnvloed deur omgewingsfaktore soos verlaagde lugtemperature asook veranderinge in substraat konsentrasies, veral verlaagde COD konsentrasies tydens die af-seisoene.. Gevolglik word die biogas en metaangas. produksie-tempo’s beïnvloed. Die teoretiese CH4 berekeninge wat gebaseer was op 15% van die damvolume asook resultate verkry tydens die biogas formasie toetse het getoon dat CH4 herwinning vanuit die anaerobiese dam ‘die moeite werd is. Die berekeninge het getoon dat daar ‘n jaarlikse minimum besparing van ongeveer R665 000 kan wees indien die herwinde CH4 tesame met steenkool gebruik word. Hierdie studie het waardevolle inligting verskaf ten opsigte van die damtoestande, mikrobiese aktiwiteit en lugtemperature wat gedurende die jaar voorkom bestaan. Inligting omtrent die biogas produksie potensiaal en CH4 produksie is verkry wat belangrik is vir die herwinning van energie..

(7) vii. Dedicated to my parents, Mr. O.P.H. Paulsen & Mrs. C.G. Paulsen.

(8) viii. ACKNOWLEDGEMENTS My sincere gratitude to the following persons and institutions that formed an integral part of this research: Dr. Gunnar Sigge and Prof. T.J. Britz of the Department of Food Science, University of Stellenbosch, as Study Leaders, for their technical support, expert guidance and overall assistance through the duration of this study; Ms. E. Muller, Appletiser South Africa (Pty) Ltd, as co-study leader, for the opportunity to undertake this study in the first place, her encouragement, professional advise, enthusiasm and mentoring throughout the completion of this study; Appletiser South Africa (Pty) Ltd, Associated Fruit Processors (Pty) Ltd and The Ernst & Ethel Erickson Trust for financial support; All the employees at Appletiser South Africa (Pty) Ltd, for their interest, participation and assistance; The Hunlun family for their loving support throughout my undergraduate and post-graduate studies; The Paulsen family, my three brothers and sister, all contributed to the successful completion of this study, through encouragement, love and support; To my parents for instilling in me the drive, courage, perseverance and passion to succeed and achieve my dreams; and The Lord for blessing me with opportunities to realise my dreams and be the most I can be..

(9) ix. CONTENTS Chapter. Page. Abstract. iii. Uittreksel. v. Acknowledgements. viii. 1. Introduction. 1. 2. Literature review. 4. 3. Establishment of an operational treatment profile for an anaerobic. 34. pond system treating fruit-processing wastewater 4. Assessment of sludge activity and biogas potential from an anaerobic. 62. pond treating fruit-processing wastewater 5. General discussion and conclusions. 98. Language and style in this thesis are in accordance with requirement of the International Journal of Food Science and Technology.. This thesis represents a. compilation of manuscripts where each chapter is an individual entity and some redundancy between chapters has, therefore, been unavoidable..

(10) CHAPTER 1. 1. CHAPTER 1 INTRODUCTION Large volumes of wastewater are generated by the food processing industry annually and consequently this results in severe environmental impacts (Trnovec & Britz, 1998). The fruit and vegetable canning industry as well as other agricultural industries are thus faced with two major problems (Sigge, 2005). Firstly, a profitable level of production needs to be maintained while reducing the intake of fresh, potable water, and secondly, the large volumes of generated wastewater need to be disposed or treated in an environmentallyfriendly manner. It has been reported that the South African apple juice processing industry is responsible for the production of 6.8 million m3 of wastewater annually (Van Schalkwyk, 2004). The peak-processing season is usually during the months of February to June when the apples are harvested and processed. Consequently, this is the period when most of the wastewater is generated. Towards the end of the peak-season (May to July) apples of a riper and softer nature are processed. The quality of these ripe apples is further influenced during the transport, washing and drying phases and the severely disintegrated apples end up in the wastewater (Van Schalkwyk, 2004).. The use of. overripe apples for juice extraction is, however, very popular since these apples cannot be sold on the fresh market but they do produce a high juice yield. Larger quantities of softer apples are thus processed during the latter periods of the processing season which subsequently leads to the production of larger volumes of wastewater that contain higher organic loads (Van Schalkwyk, 2004). These high wastewater volumes and organic loads put considerable stress on water treatment systems (Van Schalkwyk, 2004). Various wastewater treatment options exist for the treatment of fruit and vegetable wastes. Anaerobic digestion (AD) is considered to be one of the major biological treatment processes in use today (Wang et al., 1999). The AD process has several advantages over conventional disposal methods.. Large areas of land are not required (Wood, 1992);. bulking of the biomass within the digester is appreciably less than with other treatment methods (Lin & Yang, 1991); bad odours do not occur; the chemical oxygen demand (COD) is substantially lowered; nutrient requirements are low; and methane and carbon.

(11) CHAPTER 1. dioxide are obtained as final metabolic end-products (Lettinga, 2001).. 2. The methane. represents a form of combustible energy and can be used in the factory as boiler fuel, for incineration and or even to power forklifts, therefore facilitating considerable savings in energy (Lettinga, 2001). Treatment of these fruit type effluents by upflow anaerobic sludge blanket (UASB) reactors has been shown to be a feasible option. Chemical oxygen demand reductions of up to 93% at organic loading rates (OLR) of 10.95 kgCOD.m-3.d-1 and hydraulic retention times (HRT) of <12 h may be achieved while treating fruit canning wastewaters (Trnovec & Britz, 1998). Therefore, the application of the UASB design is not only applicable as a wastewater treatment system but additionally can serve as an energy recovery system. This form of biogas technology offers an attractive method of meeting partial energy needs (Yadvika et al., 2004; McGrath & Mason, 2004). It is only recently that the South African food and beverage industries have come to realise the importance of long-term incentives for the development of sustainable energy forms, such as bio-energy from anaerobic digestion of the organic rich effluents. One such apple processing company has realised the importance of waste minimisation and energy recovery and is situated in the Overberg area, South Africa. This plant processes about 70 000 tons of apples and 9 000 tons of pears annually, producing concentrates and a variety of carbonated fruit juices for the local and international markets. The production process generates large volumes of wastewater that are treated on-site via a pond and wetland system (E. Muller, Appletiser SA, South Africa, personal communication, 2004). treated wastewater is then irrigated onto the surrounding orchards.. The. The current. wastewater treatment process is considered to be efficient regarding pollutant removal. However, the possibility to produce renewable energy from the wastewater treatment process has received interest due to the economical and environmental benefits. Therefore, an investigation was set in motion to determine the feasibility of biogas recovery from an anaerobic pond which formed part of treating the apple factory wastewater. The objectives of this study were firstly, to monitor the wastewater treatment system, specifically the anaerobic pond system (Pond A) during the peak and off-seasons in order to determine and evaluate the treatment efficiencies and pinpoint any environmental limitations, and then to characterise the operational profile for the anaerobic pond. The second objective was to determine the effect of the fruit processing seasons (peak and off-.

(12) CHAPTER 1. 3. season) on the specific activity of the acidogenic and methanogenic populations in the sludge of the anaerobic pond system.. The biogas production potential at various. temperatures will also be assessed to determine the viability of CH4 recovery.. REFERENCES Lettinga, G. (2001). Digestion and degradation, air for life. Water Science and Technology, 44(8), 157-176. Lin, K. & Yang, Z. (1991). Technical review of the UASB process. International Journal of Environmental Studies, 39, 203-222. McGrath, R.J. & Mason, I.G. (2004). An observational method for the assessment of biogas production from an anaerobic waste stabilisation pond treating farm dairy wastewater. Biosystems Engineering, 87(4), 471-478. Muller, E. (2004). Appletiser SA, South Africa, personal communication. Sigge, G.O. (2005). Integration of anaerobic biological and advanced chemical oxidation processes to facilitate biodegradation of fruit canning and winery wastewaters. PhD. Thesis, University of Stellenbosch, Stellenbosch, South Africa. Trnovec, W. & Britz, T.J. (1998). Influence of higher organic loading rates and shorter hydraulic retention times on the efficiency of an UASB bioreactor treating a canning factory effluent. Water SA, 24, 147-152. Van Schalkwyk, N. (2004). The combination of UASB and ozone technology in the treatment of a pectin containing wastewater from the apple juice processing industry. MSc. Thesis, University of Stellenbosch, Stellenbosch, South Africa. Wang, Q., Kininobu, M., Ogawa, H.I. & Kato, Y. (1999). Degradation of volatile fatty acids in highly efficient anaerobic digestion. Biomass & Bioenergy, 16, 407-416. Wood, A. (1992). Anaerobic digestion in South Africa. In: Proceedings of the Third Southern. Africa. Anaerobic. Digestion. Symposium.. Pp.. 1-7.. July. 1992.. Pietermaritzburg, South Africa. Yadvika, Santosh, Sreekrishnan, T.R., Kohli, S. & Rana, V. (2004).. Enhancement of. biogas production from solid substrates using different techniques – A review. Bioresource Technology, 95, 1-10..

(13) CHAPTER 2. 4. CHAPTER 2 LITERATURE REVIEW A.. BACKGROUND. For many years the South African food and beverage industries have operated solely to be profitable and economically sustainable with very little concern for water usage, disposal of wastewater or even waste minimisation.. This mindset has led to. environmental impacts such as increased salination and eutrofication of water resources (Van Schoor, 2000).. South Africa being a water scarce country with. extended droughts and an unevenly distributed rainfall, has experienced a dramatic decline in annual rainfall patterns resulting in greater water shortages and the consequential implementation of water restrictions in certain regions of the country (Pybus, 1999). This occurrence has forced many industries to reduce water usage, implement wastewater treatment systems and introduce waste management or minimisation techniques. The fruit processing industry is responsible for the generation of large quantities of liquid and solid wastes. Thus far, current disposal practices have included treatment of wastewater in lagoons or ponds before irrigation onto land, evaporation in basins, direct irrigation and treatment through wetland systems. Some companies dispose of their effluent into municipal sewers, but current environmental legislation (Van Schoor, 2000) and the associated escalating cost of water supply and discharge has shifted the emphasis to on-site wastewater recycling and or pre-treatment before discharging into municipal sewers. Various wastewater treatment options exist for the treatment of fruit and vegetable wastes, and include both aerobic and anaerobic digestion processes (Chynoweth et al., 2001). Anaerobic digestion has been successfully implemented as an on-site secondary treatment method for fruit and vegetable wastes and offers several additional advantages over conventional methods (Azad, 1976; Drysdale, 1981; Sterrit & Lester, 1982). The most significant advantage of the anaerobic digestion process is the production of a by-product, biogas, which can be a valuable energy source..

(14) CHAPTER 2. 5. Biogas production from the biological conversion of organic waste has received increasing attention over the past few years mainly because of the ever-present risk of fossil fuel depletion (Shiralipour & Smith, 1984). The fossil fuel crisis and consequent price rises as well as the environmental impacts of fossil fuel usage have initiated the exploration of renewable energy sources (Chynoweth et al., 2001). Methane (CH4), an energetic constituent of biogas, is generated during anaerobic wastewater treatment as a by-product and can be used as fuel for boilers or reactor heating and for electricity generation (Mendonca & Campos, 2001).. Therefore, anaerobic digestion can be. considered as both a wastewater treatment process and a renewable energy production technology (Membrez & Fruteau de Laclos, 2001). Bioenergy has been cited as the most significant renewable energy source that can be used in the next few decades, at least until solar and wind power generation may offer a more economically viable large-scale alternative (Gunaseelan, 1997). The use of renewable energy from organic waste as a resource is not only “greener” with respect to most pollutants but offers an effective method for the treatment and disposal of large quantities of municipal, industrial and agricultural organic wastes. Recovering energy during such wastewater treatments might reduce the cost and somewhat reduce our dependence on fossil fuels (Angenet et al., 2004). The overall prospects of renewable energy may also offer the possibility of replacing fossil fuel derived energy and reduce environmental impacts such as global warming and acid rain (Chynoweth et al., 2001). Bio-energy and its exploitation from landfills and anaerobic digesters have been well demonstrated internationally and, to a lesser extent, in South Africa. Up to the end of 1991, the exploitation of CH4 from landfills was carried out at only two sites in South Africa, namely Grahamstown and Robinson Deep (Letcher, 1992). Over the last two decades anaerobic digestion processes have been implemented in many South African food and beverage industries as a wastewater treatment process rather than an energy recovery process. The South African food and beverage industry has recently realised that waste minimisation is the key to sustainable economic and environmental growth and development. Therefore, more and more companies are looking towards technologies offering both environmental and energy recovery advantages which comply with ISO 14 000 standards..

(15) CHAPTER 2. B.. 6. ORGANIC WASTEWATER TREATMENT METHODS. Almost all wastewaters can be treated biologically, but the wastewater must be properly analysed and the treatment process be environmentally controlled (Tchobanoglous & Burton, 1991). There are five major biological wastewater treatment processes, each varying in complexity and efficiency.. The five include aerobic, anoxic, anaerobic,. combined aerobic, anoxic and anaerobic, and pond processes. All these are based on degradation processes occurring naturally in nature (Tchobanoglous & Burton, 1991; Bitton, 1999). Aerobic Processes This biological treatment process can be subdivided into suspended-growth, attachedgrowth, and combined suspended and attached-growth processes. Suspended-growth processes include methods like activated sludge, aerated lagoons and aerobic digestion to name a few. Attached-growth processes refer to methods like trickling filters and rotating biological contactors (RBC) (Tchobanoglous & Burton, 1991; Bitton, 1999). Activated Sludge Process - The activated sludge process is mainly used as a secondary biological treatment method for various wastewaters (Bitton, 1999). This process involves the production of an activated mass of microorganisms capable of stabilising wastewater aerobically and hence its name. It involves two characteristic features namely aeration and settling (Droste, 1997; Bitton, 1999). Aerobic oxidation of the organic waste is carried out in an aeration tank. This tank contains the mixed liquor (ML), which consists of primary effluent that is mixed with return activated sludge (RAS). Aeration is provided by diffused or mechanical means and can be provided on a continuous or semi-continuous basis. It is during this time that the organic matter is oxidised to carbon dioxide (CO2), water (H2O), ammonia (NH4) and new cell biomass.. Aeration serves two purposes: the first is supplying. oxygen to the aerobic microorganisms and, secondly to ensure that the activated sludge flocs are in constant agitation to provide adequate contact between the flocs and incoming wastewater (Tchobanoglous & Burton, 1991; Bitton, 1999). Aeration is also the major energy consuming operation of the activated sludge process. The aerobic conditions also allow energy recovery in terms of biomass per unit substrate.

(16) CHAPTER 2. 7. metabolised. This results in a large quantity of excess sludge production that requires processing and disposal (Munters, 1984) and directly leads to a major operating expense and is seen as a major disadvantage of this process (Droste, 1997; Bitton, 1999; Garrido et al., 2001). The microbial cells, produced during the oxidation phase form flocs that contain large numbers of bacteria held together by secreted polymers, which accumulate on their capsules. These flocs are then allowed to settle in a clarifier. The mixed liquor is transferred from the aeration tank into the sedimentation tank where the sludge is separated from the treated effluent (Bitton, 1999). A portion of the settled sludge is recycled back to the aeration tank while the remainder is disposed of or used in further treatments such as aerobic or anaerobic digestion. Secondary clarification follows floc sedimentation. Secondary clarification is the attachment of dispersed bacterial cells and small flocs to the settled flocs.. In contrast, flocculation is a response of. microorganisms to low nutrient conditions in their environment and flocculation serves as a survival mechanism due to a more efficient utilisation of food in the close proximity of cells. The sedimentation tanks of the activated sludge process serve as a selector for microorganisms that have suitable settleability characteristics.. For instance,. organisms growing in larger floc particles settle well and are returned to the aeration tank where they are able to proliferate while smaller floc particles that are not capable of settling are not separated from the effluent and are washed out of the process (Bitton, 1999).. This is an important factor as problems can arise from insufficient. sludge separation from the treated effluent and this leads to a loss of solids (Tchobanoglous & Burton, 1991; Bitton, 1990; Contreras et al., 2000). Several modifications of the conventional activated sludge process exist and include: an extended aeration system; a complete mixed aerated system; a high rate system; a pure oxygen sequencing batch reactor (SBR); contact stabilisation; oxidation ditches; deep tanks and deep shaft processes (Tchobanoglous & Burton, 1991; Droste, 1997; Bitton, 1999). Pond Treatment Processes Waste stabilisation pond technology is a flexible treatment process, which is usually simple in design and offers economical construction and low cost operation and maintenance options (Pearson, 1996; Bitton, 1999). Modern research has allowed waste stabilisation pond technology to be expanded in terms of its application and.

(17) CHAPTER 2. 8. reliability. Pond technology may be used in the treatment of domestic sewage as well as a wide range of industrial wastewaters, and may be designed to meet exacting effluent standards (Vanderholm, 1984; Springer & Goissis, 1988; Hickey et al., 1989; (Tchobanoglous & Burton, 1991; McGrath & Mason, 2004). The existence of a wide range of pond types ensures that process optimisation is established to meet particular applications or conditions (Pearson, 1996; Bitton, 1999).. Ponds can be used to. combine wastewater treatment and storage and are considered to be the most inexpensive option for treating wastewater for subsequent re-use in aquaculture and agriculture (Hammer, 1986; Bitton, 1999).. Various pond designs can offer other. advantages such as the production of large quantities of algal biomass for animal and potential human consumption as well as energy production via methane. Ponds are thus regarded as a suitable method for wastewater and resource recovery technology compatible with current environmental thinking (Tchobanoglous & Burton, 1991; Pearson, 1996; Bitton, 1999). Pond treatment processes can be classified into four groups with respect to the presence of oxygen. These pond systems are aerobic, maturation, facultative and anaerobic stabilisation ponds (Tchobanoglous & Burton, 1991; Bitton, 1999). Aerobic stabilisation ponds – These ponds are described as large, shallow basins that are suitable for the treatment of wastewater by natural processes that involve the participation of both algae and bacteria (Bitton, 1999). The bacteria and algae are suspended in the pond whilst aerobic conditions exist throughout the depth of the pond. Furthermore, two types of basic aerobic ponds exist, both differing in objective. The operation for the first type is to maximise algae production and the design is usually limited to a depth of 150 to 450 mm. The second type is used to maximise the amount of oxygen produced and pond depths of up to 1.5 m are used. In both types oxygen or aerobic conditions are established through oxygen production via the algae and through the atmospheric diffusion of oxygen into the liquid (Bitton, 1999). Factors such as organic loading rate (OLR), degree of pond mixing, pH, nutrients, sunlight, and temperature influences the particular algal and bacterial species that are present in any section of an aerobic stabilisation pond (Tchobanoglous & Burton, 1991). In general aerobic ponds exert a high BOD5 conversion efficiency ranging up to 95%. The soluble BOD5 in an aerobic pond system will be removed from the influent wastewater but the pond effluent will contain an equivalent or higher concentration of algae and bacteria.

(18) CHAPTER 2. 9. and therefore ultimately have a higher BOD5 than the original waste stream (Tchobanoglous & Burton, 1991; Pearson, 1996; Bitton, 1999). Facultative ponds - These ponds are used for the stabilisation of wastewater by a combination of aerobic, anaerobic and facultative bacteria.. A facultative pond is. comprised of three zones (Tchobanoglous & Burton, 1991; Bitton, 1999). A surface zone is formed by a symbiotic relationship between aerobic bacteria and algae. This symbiotic relationship is responsible for the production of oxygen required by the facultative bacteria. The intermediate zone is partly aerobic and partly anaerobic and contains facultative bacteria responsible for the decomposition or oxidation of the soluble and colloidal organic materials.. The carbon dioxide produced during this. organic oxidation serves as a carbon source for the algae. Lastly, an anaerobic bottom zone exists where anaerobic bacteria decompose the accumulated solids that settle out to form the anaerobic sludge layer. This anaerobic breakdown results in the production of dissolved gases such as CO2, H2S and CH4, which are subsequently oxidised by the aerobic bacteria or vented to the atmosphere (Tchobanoglous & Schroeder, 1985). BOD removals range from 70 – 95%. Higher removal rates are obtained with longer retention times.. Facultative stabilisation ponds are conventionally used for the. treatment of screened or primary wastewater. In some cases surface aerators are used to maintain the oxygen rich surface layer. In these cases the use of algae is eliminated and as a result higher organic loads may be applied (Tchobanoglous & Burton, 1991; Bitton, 1999). Facultative ponds have been used for the treatment of food processing and canning wastes (Azad, 1976; Parker, 1978). For example it has been reported that a laboratory-scale facultative pond treating yoghurt waste removed 70 and 80% of the total and soluble COD, respectively, at retention times of 7.9 d and OLR’s as high as 450 kg COD.ha-1.d-1 (Kilani, 1992). Another laboratory-scale facultative pond system treating “synthetic” milk wastewater at loading rates of 300 kg BOD5.ha-1.d-1, achieved 90% COD removal (Al-Khateeb & Tebbutt, 1992).. An aerated/facultative lagoon. successfully treated corn wet-milling wastewater and achieved a 95% BOD removal while the system was receiving 3 450 kg COD.d-1 (Muirhead, 1990). Therefore, key applications considered for facultative stabilisation ponds are organic solids removal and pathogen removal (Mara et al., 1996; Pearson, 1996; Pescod, 1996)..

(19) CHAPTER 2. 10. Anaerobic stabilisation ponds - These ponds serve as sedimentation basins as well as anaerobic treatment devices for high-strength organic wastewaters that contain high solid concentrations (Bitton, 1999). Anaerobic ponds are usually deep earthen ponds with appropriate inlet and outlet piping. Anaerobic ponds can be as deep as 9.0 m to ensure that the anaerobic conditions are maintained throughout the pond and to conserve heat energy (Hammer, 1986; Droste, 1997; Bitton, 1999). In an anaerobic pond system organic waste is converted to CO2, CH4, and other gaseous end-products, organic acids and cell biomass. Conversion efficiencies can range from 75 to 85% when operating at optimal conditions (Drysdale, 1981; Pearson, 1996; Pescod 1996; Bitton, 1999). Anaerobic ponds have considerable advantages over facultative ponds in terms of land use and have a low surface area to volume ratio. Anaerobic ponds can sustain much higher loads than facultative ponds. Land requirements have also been greatly reduced by recent design innovations. Modern anaerobic ponds operate with minimum hydraulic retention times of one day and their inclusion in a pond system can give a land area saving of over 75% at design temperatures above 16°C (Pearson, 1996). There is no requirement with regards to expensive mechanical aeration and only small amounts of sludge are generated (Hammer, 1986; Bitton, 1999). Anaerobic stabilisation pond systems usually are operated in conjunction with other treatment processes for follow-up treatment of the partially clarified effluent (Pearson, 1996; Pescod, 1996; Bitton, 1999). The drawbacks associated with anaerobic stabilisation ponds are the production of odorous compounds (e.g. H2S), sensitivity to toxicants, and the requirement of relatively high temperatures. Anaerobic digestion of wastewater is virtually halted below 10°C. Anaerobic lagoons have been employed to treat wastewaters arising from sugarbeet processing, fruit and vegetable canning, milk processing (Azad, 1976; Drysdale, 1981; Sterrit & Lester, 1982), coffee production (Gathuo et al., 1991), slaughterhouse workings, meat and poultry processing and sugar processing wastewater (Droste, 1997). BOD removals that have been achieved, have ranged from 20 - 95% at BOD loading rates of up to 3 360 kg.ha-1.d-1 were recorded (Azad, 1976; Droste, 1997). Pearson (1996) also emphasised the role that waste stabilisation pond technology plays in energy recovery. He reported that CH4 recovery from facultative pre-treatment ponds was possible using submerged gas collectors. The CH4 recovered from pond systems is of a good quality and not only represents energy recovery but.

(20) CHAPTER 2. 11. also prevents the release of CH4 into the environment. Conventional pond systems are seen as economical because of lower costs in terms of construction, operation and maintenance. The microbiological quality of the effluent treated using pond systems is such that it is suitable for the use in unrestricted irrigation. This is allowed through a natural disinfection process, without the addition of chemicals or the use of physical disinfection technology. Any other form of wastewater treatment technology cannot match this important characteristic. The water, nutrients and algae in pond effluents all represent energy recovery when used directly for crop irrigation (Pearson, 1996). Anaerobic digestion Anaerobic digestion (AD) is the decomposition of organic material and the subsequent formation of CH4 and CO2 by microorganisms in the absence of oxygen (Bryant & McInerney, 1981; Chynoweth et al., 2001; Mata-Alvarez, 2003). Anaerobic digestion relies on the presence and activity of specific bacterial species that form a community, each with specialized ecological roles and complex nutritional requirements (Bryant & McInerney, 1981; Iannotti et al., 1987; Barnett et al., 1994). Four different bacterial groups as shown in Fig. 2.1 with their synergistic relationship are known to be responsible for the AD process (Bryant & McInerney, 1981; Blaut, 1994; Mata-Alvarez, 2003). Hydrolytic and fermentation bacteria (Acidogens) - The acidogens catabolize carbohydrates, proteins, lipids and other minor components of complex organic molecules to soluble monomer molecules such as amino acids, glucose, glycerol, short chain fatty acids, hydrogen and carbon dioxide by a process of hydrolysis (Gander et al., 1993; Mata-Alvarez, 2003). Hydrolysis of the complex molecules is governed by extracellular enzymes, especially the hydrolases. Depending on the type of reaction catalysed, the extracellular enzymes can also include cellulase, protease and lipases (Gander et al., 1993). The soluble monomers are readily available to microbial cells and can be metabolised (Forday & Greenfield, 1983; Bitton, 1999).. The principal. volatile fatty acids that are formed include acetic, propionic and butyric acids, however, small quantities of valeric acid may also be present (Chen et al., 1980; Duarte & Anderson, 1982; Forday & Greenfield, 1983; Batstone et al., 2002; Mata-Alvarez, 2003). The main reactions take place as follows:.

(21) CHAPTER 2. 12. C6H12O6 + 4H2O → 2CH3COO- + 2HCO3- +4H+ + 4H2 glucose. acetate. C6H12O6 + 2H2 → 2CH3CH2COO- + 2H2O + 2H+ glucose. ∆G° = -198 kJ. lactate. C6H12O6 → 2CH3CH2OH + 2HCO3- + 2H+ glucose. ∆G° = -358 kJ. propionate. C6H12O6 → 2CH3CHOCOO- + 2H+ glucose. ∆G° = -206 kJ. ∆G° = -226 kJ. ethanol. During this phase acidification occurs but the chemical oxygen demand (COD) reduction is minimal. However, some COD reduction may occur when large amounts of H2 and CO2 are produced but this COD reduction is seldom higher than 10% (Noike et al., 1985).. Hydrolysis can be considered as the rate-limiting step in the overall. anaerobic digestion process because it is a relatively slow step (Noike et al., 1985). This can be especially true for wastes such as raw cellulitic wastes containing lignin (Bitton, 1999). The efficiency of the hydrolysis step also directly contributes to the ultimate CH4 yield. Acetogenic Bacteria - The hydrogen-producing acetogenic bacteria catabolize certain fatty acids and neutral end-products to acetate, formate, CO2 and H2. Other fatty acids and metabolites such as propionate, butyrate, lactate, succinate and alcohol may also be produced from odd-numbered carbon skeletons (Chen et al., 1980; Zinder, 1990). Ethanol, propionate and butyrate are converted to acetate according to the following reactions (Forday & Greenfield, 1983; Bitton, 1999; Batstone et al., 2002; Mata-Alvarez, 2003): CH3CH2OH + H2O → CH3COO- + H+ + 2H2 ethanol. acetate. CH3CH2COO- + 3H2O → CH3COO- + H+ + HCO3- + 3H2 propionate. ∆G° = +9.6 kJ. acetate. ∆G° = +76.1 kJ.

(22) CHAPTER 2. 13. Composite particulate waste and inactive biomass. Inert particulate. Disintegration. Inert soluble. Carbohydrate. Proteins. Fats. Hydrolysis. AA. MS. LCFA. Acidogenesis. HVa, HBu. Propionate. Acetogenesis. Actetate. H2. CH4, CO2. Methanogenesis. Figure 2.1 Outline of the conversion processes in anaerobic digestion according to the Anaerobic Digestion Model No. 1 (ADM1) (Batstone et al., 2002). Abbreviations include MS (monosaccharides); AA (amino acids); LCFA (long chain fatty acids); HVa (valeric acid); HBu (butyric acid)..

(23) CHAPTER 2. 14. CH3CH2CH2COO- + 2H2O → 2CH3COO- + H+ + 2H2 butyrate. ∆G° = +48.1 kJ. acetate. CHCHOHCOO- + 2H2O → CH3COO- + HCO3- + H+ + 2H2 ∆G° = -4.2 kJ lactate. acetate. 3CHCHOHCOO- → 2CH3CH2COO- + CH3COO- + HCO3- + H+ lactate. propionate + acetate. ∆G° = -165 kJ. CHCHOHCOO- + 2H2O → CH3CH2CH2COO- + 2HCO3- + 2H2 lactate. butyrate + acetate. ∆G° = -56 kJ. Acetogenesis can only occur if the H2 partial pressure concentration is very low and therefore acetogens can only proliferate if H2-reducing bacteria are present, thus distinguishing them from homoacetogenic bacteria (Ianotti et al., 1987; Bitton, 1999). Homoacetogenic Bacteria - The homoacetogenic bacteria synthesise acetate using CO2 and H2 and formate or hydrolyse multicarbon compounds to acetic acid. Degradation of the fatty acids also requires low H2 tensions (Bitton, 1999). Therefore, these bacteria are equipped with highly efficient hydrogenases, which have a high affinity for their substrate and maintain an exceptionally low H2 partial pressure during active methanogenesis (Chen et al., 1980; Varnam & Evans, 2000). If a relatively higher H2 partial pressure exists, acetate formation is reduced and the substrate is converted to propionic and butyric acids and ethanol rather than CH4. It is therefore, the symbiotic relationship between the acetogenic bacteria and methanogens that is of significance during this process. The methanogens are responsible for achieving the low H2 tension, which is required by the acetogenic bacteria. Methanogenic Bacteria - The methanogenic bacteria most generally utilise acetate, CO2 and H2 to produce CH4 (Chen et al., 1980; Jetten et al., 1992; Fang, 2000). This group of bacteria are obligatory anaerobes requiring a low oxidation reduction potential (-300 mV) for growth. They are comprised of Gram-positive and negative bacteria with a diverse morphology (Forday & Greenfield, 1983; Ianotti et al., 1987). Furthermore,.

(24) CHAPTER 2. 15. two main sub-categories exist for the methanogens. The first is the hydrogen utilising group or hydrogenotrophic group that splits acetate and convert H2 and CO2 to CH4 as follows (Blaut, 1994): HCO3- + 4H2 + H+ → CH4 + 3H2O. ∆G° = -135.6 kJ. This methanogenic group is also responsible for maintaining the very low H2 partial pressure, which is necessary for the conversion of volatile fatty acids and alcohols to acetate (Ianotti et al., 1987; Bitton, 1999). The second group, the acetotrophic methanogens or acetoclastic methanogenic methanogens, are responsible for the conversion of acetate to CH4 and CO2 as follows (Zinder, 1993; Bitton, 1999): CH3COO- + H2O → CH4 + HCO3-. ∆G° = -31 kJ. This group grows much slower than the acetoclastic methanogens, with doubling times of 2 –12 days. They are responsible for the generation of at least two-thirds of the CH4, generated during the conversion of acetate (Jetten et al., 1992). The remaining third is the result of CO2 reduction by H2 (Ditchfield, 1986; Mackie & Bryant, 1981). C.. IMPACT OF ENVIRONMENTAL FACTORS. The efficiency of the AD process is governed by specific conditions that must be favourable to the bacterial consortium.. These specific conditions include the pH,. temperature, alkalinity, volatile acid concentration, nutrient availability, retention time, competition of methanogens with sulphate-reducing bacteria, composition of the wastewater and absence of toxic materials (Chen et al., 1980; Bryant & McInerney, 1981; Forday & Greenfield, 1983; Mata-Alvarez, 2003). pH - Research has shown that one of the most important environmental factors associated with optimising methanogenic production of biogas is the system pH (Mawson et al., 1991; Speece, 1996). The methanogens and acidogens are extremely sensitive to pH changes and in most cases have an optimum pH growth range of 7.07.4. Similarly for optimum CH4 production, a pH range between 6.7 and 7.4 is essential.

(25) CHAPTER 2. 16. (Chen et al., 1980; Bryant & McInerney, 1981; Koster, 1986; Mata-Alvarez, 2003). Acute toxicity occurs when the pH level falls below 6.0 and this is usually associated with the presence of undissociated volatile fatty acids produced by the acidogenic bacteria (Chen et al., 1980; Wang et al., 1999; McMahon et al., 2001). The pH is also a function of the bicarbonate alkalinity, CO2 partial pressure and the concentration of volatile acids (Chen et al., 1980; Nel & Britz, 1986; Mata-Alvarez, 2003). Methanogens are more susceptible to acidity than the acidogenic bacteria and thus an increase in volatile acid concentration may indicate system upset (Bitton, 1999). The maintenance of an acceptable pH range is brought about by the combined activities of the acetogenic and methanogenic populations. Bicarbonate produced by methanogens serves as a buffering agent during pH reduction (Bitton, 1999; Mata-Alvarez, 2003). Alkalinity - The alkalinity of an anaerobic system has been shown to be important as it is a measure of the buffering capacity that in an AD normally may consist of bicarbonate, carbonate, ammonia, phosphate and hydroxide components (Ahring, 1995). Organic acids and acids salts may also contribute to the buffering capacity in anaerobic systems. A bicarbonate alkalinity in the range of 2.5 to 5.0 g of CaCO3.L-1 provides sufficient and a safe buffering capacity for the anaerobic treatment of waste (Chen et al., 1980; Mata-Alvarez, 2003). Organic acids - The organic acid level of anaerobic systems is also an important determinant of the digestion efficiency (Ahring, 1995; Wang et al., 1999). It has been well documented that the organic acid level should remain below 2000 mg.L-1 of acetate for efficient digestion and that higher levels have been shown to be toxic (Chen et al., 1980; Mata-Alvarez, 2003). Acute methanogenic toxicity occurs at unionised volatile acid concentrations of between 30 and 60 mg.L-1 as acetic acid and this, under certain conditions corresponds to a total volatile acid concentration of between 1 650 – 2 600 g.L-1 (Chen et al., 1980; Duarte & Anderson, 1982; Mata-Alvarez, 2003). At this concentration the pH is usually well below 6.3 (Taconi, 2004). Temperature - Temperature is another important environmental factor that will negatively impact the anaerobic digestion processes. Two optimum temperatures have been reported for the anaerobic treatment of organic wastewater. The optimum for mesophilic anaerobic digestion is 35°C (usually ranges between 25° and 40°C) and for.

(26) CHAPTER 2. 17. the thermophilic is 60°C (usually ranges between 50° and 65°C) (Patel & Madamwar, 1984; Uemera & Harada, 1995; Bitton, 1999). It has also been shown that it is less expensive to produce CH4 at higher temperatures.. Higher temperatures during. digestion give rise to faster fermentation rates, which directly impact the loading rates and also leads to a minimisation of bacterial and viral pathogens (Bitton, 1999). Increases in temperature, especially between 35° and 65°C, may also cause increases in total volatile acid concentrations and therefore induce a higher sensitivity to toxicants (Bitton, 1999). At very low temperatures (below 10°C) very little CH4 is produced. The microbial cells, however, remain viable and continue to grow and will actively produce CH4 when the incubation temperature is increased to 35°C. It has been reported that during incubation of mesophilic methanogens at 45°C no growth or CH4 production occurs and the cells were found to become non-viable after four weeks of incubation at this temperature (Patel & Madamwar, 1984; Taconi, 2004). Nutrients - To maintain the anaerobic digestion process, the wastewater being treated must be nutritionally balanced (Bitton, 1999).. The presence of nutrients, such as. nitrogen, phosphorus and sulphur as well as other trace elements needed by the bacteria play a crucial role in this process (Speece, 1996). The best C:N:P ratio for the AD process is recommended to be 700:5:1 (Bitton, 1999; Mata-Alvarez, 2003). It has also been suggested that for optimal gas production the C:N ratio should be 25-30:1 (Bitton, 1999; Mata-Alvarez, 2003).. Some minerals have been found to exhibit a. stimulatory effect at low concentrations and these include sodium, potassium, calcium, magnesium and iron (Bitton, 1999; Azbar & Speece, 2000; Mata-Alvarez, 2003). In contrast, at higher concentrations these elements may show inhibitory effects (Bitton, 1999; Mata-Alvarez, 2003). High concentrations of sulphate are known to especially retard the production of methane.. The mechanism by which sulphate inhibits. methanogenesis is by the competition for the available H2. The sulphate-reducing bacteria are able to scavenge the available H2 faster than the CH4 bacteria (Lusk, 1998). Therefore, this results in the shunting of electrons from the CH4 generation pathway to sulphate reduction (Bitton, 1999). Methanogenic bacteria, in general, have simple nutrient requirements and those species that require organic materials such as B-vitamins, fatty acids and amino acids for growth, obtain it from other bacterial species that produce them during wastewater catabolism (Jarrel & Kalmokoff, 1988; Bitton, 1999; Mata-Alvarez, 2003)..

(27) CHAPTER 2. 18. HRT - The hydraulic retention time is another important operational factor in the anaerobic digestion process. The HRT is dependent on the wastewater properties, carbon concentration and environmental conditions and must be long enough to allow for sufficient digestion of the waste by the anaerobic bacteria (Bitton, 1999). The HRT usually varies with the different types of digesters and is also directly dependent on the operational temperature (Bitton, 1999; Mata-Alvarez, 2003). It usually varies from 10 h to several days (10 – 30). In contrast, in anaerobic digestion systems the minimum solids retention time (SRT) is in the range of 2 - 6 days, depending on the temperature (Forday & Greenfield, 1983). D.. MANAGEMENT OF THE ANAEROBIC DIGESTION PROCESS. From a superficial point of view, biomethanisation appears to be a simple process because it looks as if it only consists of two-steps. As described earlier, the first major step involves the conversion of organic matter to intermediates such as VFAs, CO2 and H2. During the second major step the intermediates are metabolised into CH4 by the methanogens (Mata-Alvarez, 2003). However, these two major steps of the anaerobic digestion process are very sensitive to disturbances in the environment (Table 2.1) and even minor changes can result in digester organic overload and subsequent process failure (Mata-Alvarez, 2003).. Causes of digester overload can be ascribed to the. substrate containing excess organic biodegradable compounds as well as any environmental occurrence that will result in a decreased concentration of active microorganisms.. These can include dramatic organic loads, temperature and pH. changes, toxic substances introduced into the system and sudden increased flow rates (Mata-Alvarez, 2003).. Under disturbed operational conditions the acidogens show. more tolerance and continue to function, metabolising and producing more acids. The methanogens on the other hand are negatively affected by disturbances in their environment and their activity decreases (Mata-Alvarez, 2003).. The discrepancy. between the activity of the acidogens and methanogens during adverse conditions results in an antagonistic relationship. The higher concentration of acids produced by the acidogens are not removed and thus directly inhibits the activity of the methanogens and subsequently other intermediates are formed, in order to metabolise the. accumulated. H2. and. formate. (Mata-Alvarez,. 2003)..

(28) CHAPTER 2. Table 2.1.. 19. Examples of anaerobic digester operational parameter changes and their impacts (Mata-Alvarez, 2003).. Effect on microbial. Impact on digester. population. efficiency. Decrease in concentration of. Reduced: % CH4 in. microorganisms; methanogens. biogas; lower pH and. growth inhibited - long. alkalinity; lower CH4. doubling time. production rate. Acidogens produce more or. pH declines. less acids;. Alkalinity declines. Increased feed. Methanogens affected;. Increased VFA. concentration. acidogens produce more acids. concentration; lower pH. that methanogens cannot. and alkalinity; other. remove. metabolic pathways. Parameter change. Increased flow rate. Change in feed type. needed to degrade compounds.. Presence of toxic. pH and alkalinity influenced. Consortium balance. substances. Specific population. destroyed. destroyed/damaged or inhibited Temperature fluctuations. Microbial activity decreases;. Consortium balance. might die off. destroyed.

(29) CHAPTER 2. 20. Consequently, the pH is lowered and the alkalinity levels decline and digester failure is inevitable if this imbalance persists. In order to overcome problems that may arise as a result of operational disturbances such as feeding high strength waste at short HRTs, the management or control of the digester needs to be sustained. An anaerobic digestion control system must be set up so as to detect disturbances and even be used to calculate indirect variables from direct measurements.. The “health state” of the system should be. calculated by measuring process variables such as substrate concentration, flow rate, temperature, hydrogen pressure, gas levels and more specific measurements like pH, alkalinity and VFAs (Mata-Alvarez, 2003). The process stability and efficiency can therefore be established by regularly monitoring the variables as given in (Table 2.2). As some of these variables are more sensitive to disturbances, an early indication of process imbalance must be identified as soon as possible (Mata-Alvarez, 2003). The control of the anaerobic digestion process is essential in establishing efficient handling of unstable processes and for managing and maintaining the operation of systems at optimal conditions (Mata-Alvarez, 2003). E.. APPLICATION OF ANAEROBIC DIGESTION TO DIFFERENT WASTEWATERS. Anaerobic digestion has in the past been used for the dual purpose of treating wastewater and for energy conservation in the form of CH4 recovery. The treatment of a variety of wastewaters has been studied for many years. In 1986, Koster reported that when using an Upflow Anaerobic Sludge Blanket (UASB) reactor to treat an acidic tomato waste stream to produce methane, as much as 80% of the influent COD was converted to methane.. The highest loading rate obtained in this case was 22. kgCOD.m3.d-1 and the treatment efficiency resulted in a 90% COD reduction. Hemming (1981) also reported that biogas containing 70% CH4 was produced in a full-scale anaerobic digestion plant treating potato processing effluent. The BOD concentration was reduced by 60% at an HRT of 7 d and a total biogas production of 1 800 m3.d-1. Szendrey & Dorion (1986) reported the successful operation of a 13 million litre fixed-film downflow anaerobic filter while treating distillery wastewater. -3. study an optimum loading rate of 12.84 kg COD.m .d. -1. During this. was achieved with a. wastewater pH range of 7.2 - 7.4. The volatile fatty acid concentration was considered to be the most important operational parameter and ranged between 3 000 and 4 000 mg.L-1. The average biogas production was 0.56 m3.kgCOD-1removed during the full-.

(30) CHAPTER 2. Table 2.2.. 21. Operational variables important for controlling the anaerobic digestion process (Mata Alvarez, 2003).. Liquid phase. Gas phase. VFA (type and concentration). Gas production rate. pH (low, optimum, high). CH4 / CO2 production rates & concentrations. Alkalinity (low, high). CO content in biogas. OLR. Gas pressure. Nutrient level and additions. H2 content in biogas. HRT. Temperature. C:N:P ratios. Gas phase volume. Temperature COD (type and concentration) Liquid level.

(31) CHAPTER 2. 22. scale study. They also reported that the CH4 content (50 – 65%) of the biogas was sensitive to the pH and volatile acid concentrations. Nand et al. (1991) used canteen and mess wastewater in a study to determine their biogas generation potential. The anaerobic digestion was performed in a floating dome type digester and certain of the environmental and operational parameters influencing the biogas yield and the CH4 content were optimised. During this study a high gas yield of 0.981 m3.kg-1 VSadded was obtained with a CH4 content of 50% and 65% substrate utilisation. The gas yield was directly proportional to the HRT at 27° 33°C, and an OLR of 100 kg total solids m-3.d-1 was achieved. Anaerobic treatment of winery and distillery wastewaters has also been researched (García-Bernet et al., 1998; Ronquest & Britz, 1999; O’Kennedy, 2000; Ruiz et al., 2002). O’Kennedy (2000) used a mesophilic lab-scale UASB reactor to treat high strength distillery effluent. The average COD removed was >90% at an OLR of 30 kgCOD.m-3.d-1 with a pH of 7.8 and biogas production of 18.5 L.d-1. The anaerobic treatment of various fruit and vegetable wastewaters has also been documented. Viswanath et al. (1992) determined the effect of different fruit and vegetable wastewater on digester performance in terms of biogas production in a 60 L digester. Different OLRs and different HRTs were studied. The maximum biogas yield was 0.6 m3.kg-1 VSadded at a 20 day HRT and an OLR of 40 kgTS.m-3.d-1. Austermann-Haun et al. (1994) treated fruit juice factory effluent using an UASB reactor. A space loading rate of 3.6 kgCOD.m-3.d-1 and an influent COD concentration of 2 337 mg.L-1 was used. The COD removal efficiency achieved was 80 – 90%. Trnovec & Britz (1998) also treated fruit canning wastewater using a mesophiliclaboratory scale UASB reactor. The OLR was 10.95 kgCOD.m-3.d-1 with COD removals of 90 – 93 % and HRT of 10 h. More recently Britz et al. (2000) reported that an UASB reactor treating fruit cannery wastewater devoid of lye, can achieve OLR’s of 9.2 kgCOD.m-3.d-1 with and average COD removal of 81 – 84 % and HRT of 10 h. The influent COD levels ranged between 3 800 – 4 300 mg.L-1 at an influent pH of 5.0. Some effluents are an ideal substrate for UASB digestion, like wastewater originating from the sugar beet industry (Van Lier et al., 2001).. However, other. wastewaters that contain complex organic material may not be treated as easily using the UASB process (Lettinga et al., 1980). Problematic constituents such as pectin found in effluents originating from the fruit processing industry will influence the process and could lead to various problems in the UASB reactor (Fedirici et al., 1988). Other.

(32) CHAPTER 2. 23. effluents such as apple juice processing wastewater require pH adjustment before the UASB treatment process can proceed. This is due to the wastewater’s low pH range (pH 3 – 5) (Wayman, 1996). However, Britz et al. (2004) showed that the use of ozone as a pre-treatment improves the suitability of apple juice processing wastewater for UASB degradation by breaking the pectin gel that would otherwise interfere during normal UASB digestion. F.. BIOGAS FROM ANAEROBIC DIGESTION. Anaerobic digestion of biodegradable waste results in both potential energy generation and reduction of greenhouse gas emissions (Baldasano & Soriano, 2000; Keller & Hartley, 2003).. Methane, an energetic constituent of biogas, is generated during. anaerobic wastewater treatment as a by-product and can be used as fuel for boilers or reactor heating and for electricity generation (Mendonca & Campos, 2001). Biogas is a renewable source of energy with much lower environmental impacts than conventional fossil fuel (Fan et al., 2001; Ho, 2005). Biogas – It has been reported that anaerobic digestion of organic waste releases between 500 – 800 million tons CH4 annually into the atmosphere (Bitton, 1999). Biogas is comprised of four major constituents: methane, carbon dioxide, moisture and hydrogen sulphide.. The concentrations are given in Table 2.3.. The normal. composition of biogas from efficient operating anaerobic digestion systems ranges from 60 – 70% CH4 with the balance being CO2. The energy content of biogas is entirely associated with the CH4, which has an energy value of 37 MJ.m-3. The amount of CH4 that can be produced during the anaerobic digestion process from organic material is directly proportional to the substrate content of convertible COD. Since no oxidation by atmospheric O2 can occur the biodegradable COD from the substrate will be preserved in the end-products. Stoichiometrically CH4 has a COD of 2 moles (= 64 g of COD) of oxygen per mole (= 16 g) of CH4. Thus 1g of CH4 is equivalent to 4 g of COD. Further calculations show that from 1 Kg of COD 0.355 m3 CH4 at STP can be produced which is equivalent to 14 132 kJ usable energy (as CH4). Methane has a critical temperature and pressure of - 82ºC and 4.6 MPa, respectively (Chen et al., 1980; Chynoweth et al., 2001; Mata-Alvarez, 2003). It is not possible to liquefy or compress CH4 at higher temperatures. (Mata-Alvarez,. 2003).. Biogas. can. be.

(33) CHAPTER 2. 24. Table 2.3. Composition of Biogas.. Constituent. Value (range). Methane. 50 - 65%. Carbon Dioxide. 35 - 50%. Moisture. 30 - 160 g.m-3. Hydrogen Sulphide. 1.52 - 12.5 g.m-3.

(34) CHAPTER 2. 25. stored for long periods at reasonable costs this property allows for the conversion of biogas into electricity during hours when the electricity is expensive or periods of high electricity demands (Mata-Alvarez, 2003). Carbon dioxide is the other major constituent of biogas. Its characteristics are dependent on different factors such as substrate composition, pH, reactor pressure, temperature, HRT, process design, etc (Mata-Alvarez, 2003). The effect of CO2 in biogas is that it dilutes the energy value of biogas and increases the volume to be handled and stored. Alvarez, 2003).. CO2 removal is not necessary for thermal application (Mata-. However, when feeding gas into a public gas pipeline and for its. storage under pressure (fuel for cars) the CO2 has to be removed (Mata-Alvarez, 2003). Biogas also contains water vapour, which is an important contaminant that should be removed before application (Table 2.3). Condensed water poses a problem when it accumulates in gas handling equipment and meters. This causes problems such as frozen pipes and corrosion of metal parts when combined with the H2S also present in the gas (Chynoweth et al., 2001). The water can be condensed by watertraps (Mata-Alvarez, 2003). Biogas has to be purified in order to meet natural gas pipeline standards. The most economical and commercial cleaning process used is water scrubbing (Chynoweth et al., 2001). Other methods such as the phosphate buffer and membrane separation processes are comparable in cost but neither has been extensively fieldtested. Water scrubbing is a relatively simple process in which the H2S-free biogas is compressed and then flows “counter current” to water in a pressurized packed column. The scrubbed CO2 is vented to the atmosphere and the water recycled back to the stripping column.. The scrubbed biogas is then dehumidified to meet natural gas. pipeline standards. H2S can be removed with a simple and common method called the iron oxide (or iron sponge) process (Chynoweth et al., 2001). Biogas Carbon Trading - The benefits of biogas recovery from anaerobic digesters to rural communities have been well demonstrated in developing countries such as India, China, Sri Lanka and Nepal (Ho, 2005). The rural energy benefits provided by biogas recovery includes energy for cooking and lighting. Other benefits of promoting biogas recovery have to do with “carbon trading”. The United Nations Framework Convention on Climate Change has set up a “Clean Development Fund”, and the World Bank has provided a “Carbon Finance Unit”, which allows developed countries to buy emissions.

(35) CHAPTER 2. 26. when more carbon is released into the atmosphere than what is allowed according to the Kyoto Protocol from developing countries.. This will prevent carbon emissions. through conserving forests and renewable energy (Ho, 2005). Internationally, Nepal’s biogas programme is regarded as a model for the successful use of alternative energy for the rural Third World. Nepal has 125 000 functioning anaerobic digesters treating various domestic, agricultural and industrial wastes which prevents 5 tonnes of carbon dioxide equivalents annually from being emitted into the atmosphere.. This amounts to US$ 5 million in greenhouse gas. emissions that can be traded (Ho, 2005). This money can be invested back into clean energy production, therefore preventing even more greenhouse gas emissions. Biogas recovery from anaerobic digesters yields resources with significant financial and intangible value. Community biogas plants, especially in rural Third World countries, may serve as the most useful decentralised sources of energy supply and may to some degree reduce our dependence on fossil fuels (Angenet et al., 2004; Ho, 2005). G.. DISCUSSION. In order to overcome the major problem of water scarcity and processing effluent disposal in the South African fruit processing industry, initiatives such as on-site wastewater treatment technologies and energy recovery technologies should be undertaken. Biological treatment is one of the most efficient and economical ways to reduce high COD values. Aerobic digestion is the traditional and most widely used option to treat oxygen consuming wastewater. Various problems are, however, associated with aerobic processes such as high energy requirements (>100 kW.ton-1 of COD reduced) and the production and disposal of large sludge quantities (50% of the COD). Anaerobic treatment of wastewater may be seen as a technology which is an energyefficient approach to waste management (Bitton, 1999). A positive energy balance due to the production of biogas and low sludge volume productions are of the main advantages of AD. Anaerobic digestion can be seen as a mature technology and many successful systems have been installed worldwide. On-site anaerobic treatment of effluents offers several advantages to the industries. It allows for water re-use, therefore enabling lower capital outlays for water.

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